The present invention relates to a method and apparatus for producing moulds or mould parts In a mould-forming machine comprising at least one moulding chamber with at least one movable squeeze plate and at least one pattern, such as a string moulding-apparatus, a flaskless mould-forming machine or the like, in which the mould-forming process comprises the steps of filling the moulding chamber(s) with compressible mould material, e.g. clay-bonded green sand, applying a vacuum at selected parts of the pattern(s) during said filling, and pressing the mould material by moving the squeeze plate(s), thus forming the mould or mould parts (cope and drag), the moulding cavities being provided in the produced mould or mould parts by means of the pattern plate(s). After pressing the mould material, the produced mould or mould parts are removed from the moulding chamber(s), whereafter the moulding chamber is reclosed and made ready for a new cycle.
A method and apparatus of this general kind is e.g. known from U.S. Pat. No. 4,791,974. According to this document, the filling of the moulding chamber with compressible mould material, e.g. clay-bonded green sand, is improved by applying a vacuum to selected parts of the pattern(s) through air-permeable chamber walls during the whole filling step and possibly before starting the filling, thus avoiding the formation of air pockets in depressions in the pattern(s) that could otherwise cause reduced compactness and density in protruding parts of the mould or mould parts being formed in the moulding chamber. In the above document, the vacuum is applied during the whole filling step, whereby the surface of the produced mould or mould parts is dried out and the amount of air to be removed is relatively high, resulting in a demand for relatively large channels for removing the air. Furthermore, the drying-out of the surface of the produced mould or mould parts is effected in positions where this drying out is highly unwanted and which produces a difference in quality of the mould surface and accordingly the surface of the moulded products produced in the moulds. U.S. Pat. No. 5,332,025 correspondingly discloses a method and apparatus for filling sand moulds in a string moulding machine in which vacuum or gas pressure can be applied to the sand in the moulding chamber. A valve connects the moulding chamber either to a pressure or vacuum source. The application of vacuum results in the same disadvantages of drying out the mould, as described above for U.S. Pat. No. 4,791,974.
It is the object of the present invention to provide a method and an apparatus, which improves the mould quality of a mould-forming machine of the kind referred to above, which alleviates the above-mentioned problems, thus resulting in a higher quality of the produced moulds and a more efficient utilisation of the vacuum. This object is achieved with a method and apparatus of said kind, wherein vacuum can be applied separately to different parts of the pattern(s), during such periods of times during which the vacuum is efficient to improve the filling of said different parts. Preferred embodiments of the invention are also revealed hereinafter.